Typical electrical shielded cables of this type have four twisted-pairs of wires surrounded by a helically wrapped shield or a longitudinally wrapped shield and a generally longitudinally extending drain wire. The various construction of known cables are illustrated in the 1993 Cooper/Belden catalog pages 52-60, 78-83, 105, 109, 110, 165-169, 240, 251, 257-260, 278. These generally show a lateral aluminum-polyester shield with a longitudinally extending overlap surrounding a plurality of insulated twisted pairs of wires. Also, the cables have a longitudinally extending drain wire. Other manufacturers are known to also use helically wound metal-polyester shields.
The problem with the prior art construction for certain types of applications is that it is difficult to produce a cable that will have sufficient flexibility and hoop strength for ease of installation such as being pulled through various conduit angles and still maintain a relatively low impedance variation throughout the length of the cable. The impedance variation is improved by providing uniform shield to conductor spacing along the length of the cable, which results in improved electrical properties along the length of the cable. The spiral drain also improves shorting at the fold.
In prior art spiral shield designs, impedance instability is brought about by a loosening of the shield where it is overlapped and a kinking of the shield when it is drawn through various types of conduits. Spiral shields tend to conform to the core elements geometry and are not nearly as geometrically stable as a lateral corrugated shield and drain. This causes the spacing of the individual conductors to the wrapped metal foil shield to vary along the cable length causing the cable electricals to vary with frequency.
Also, the spiral wrapped shields used in high frequency cables, generally must have a relatively large overlap--as much as 25% in some instances to prevent the shield from leaking. Leakage around a spiral slot creates an inductor which sets up a circumferential electrical field. This electrical field radiates interference. Leakage around a lateral or longitudinal slot radiates interference less effectively in that lit radiates generally in one plane rather than the radiation of a spiral slot which radiates as much as 360.degree., and as a result, does not inductively couple interference ingressively or egressively.
It is an object of the present invention to substantially reduce the problems of the prior art in cables and especially in high frequency transmission cables.
It is therefore an object of the present invention to provide a cable having a group of insulated conductors which have at least one pair of insulated conductors, a lateral shield wrapped around the group of conductors, said shield being overlapped longitudinally along the length of the cable and the shield having a metal conductor surface facing a cable jacket, a drain wire helically wrapped around the shield and being in electrical contact with the metal conductor surface of the shield, the drain wire forms a helical corrugation or groove in the shield conductor surface, and an insulator jacket covering the drain wire and the shield.
It is a specific object Of the present invention to provide a high frequency cable wherein the cable has an insulator jacket extending the length of the cable, a group of conductors which have at least one twisted pair(s) of conductors, each twisted pair of conductors having the dielectric insulating layer surrounding each conductor with the dielectric layers being joined or unjoined together along the length of the conductor, a lateral shield wrapped around the group of twisted pair conductors and extending the length of the cable, the shield having a metal conductor surface facing the jacket, the thickness of the shield being between about 0.5 to about 4.0 mils, a metal drain wire helically wrapped around the shield so as to produce spacing between each helical interval of from about 0.125 to about 0.75 inches, the drain wire providing a helical groove in the shield with the depth of the groove being at least 50% of the diameter of the drain wire and the insulating jacket is wrapped over the drain wire and shield and is optionally sized such that the outer surface of the jacket has a helical protuberance which corresponds to the contours of the helical drain wire.
Another specific object of the present invention is to provide a high frequency cable having a group of plurality of twisted pair conductors and a longitudinal drain wire laterally wrapped with a metal-plastic shield having a conducting surface facing the twisted pair conductors and a non-conducting surface facing the outer cable jacket, a non-conducting cylindrical plastic or synthetic cord helically wrapped around said shield and extending the length of the cable, the cord forms a helical groove in the shield with the helical spacing interval being between about 0.125 to about 0.75 inches, the depth of the groove being at least 50% of the diameter of the cord, the jacket is optionally sized such that the outer surface of the jacket has a helical protuberance which corresponds to the contours of the helical cord.
The present invention provides an improved cable by substituting a lateral shield having a helical groove for the shields of the prior art.
Generally, in high frequency cables, where the invention generally finds an economical and performance advantage, the shields are made of metal such as aluminum, copper and suitable metal alloys. Other metals, such as zinc may be used providing they are sufficiently flexible to form the required helical groove. The preferred shield has a conductor on one surface and a non-conductor on the opposite surface. The Beldfoil.RTM. shield of Belden Wire & Cable Company is such a shield.
The conductive surface of the metal-plastic shield is aluminum, copper, zinc, or an appropriate conductive metal alloy. The non-conductive plastic surface is generally a polyester. The thickness of the shield for high frequency cables can be from about 0.5 to about 4 mils. The preferred metal is copper or aluminum and the preferred thickness of the shield is about 1 to about 3 mils.
However, in certain applications such as electrical grade cable, an armor type shield of steel with a thickness of from about 4 to about 6 mils is contemplated.
The shield is laterally wrapped over the group of insulated conductors with the ends or sides overlapped and the overlapping ends extending longitudinally for the length of the cable.
The shield having its conductive surface facing away from the group of insulated conductors is helically wrapped with a drain wire. The wrapping is sufficiently tight to form a corresponding helical groove or corrugation in the shield. The diameter of the drain wire is from about 0.015" to about 0.050". The drain wire can be made of any appropriate material. However, it must be sufficiently strong and flexible to be wrapped around the shield, holds its configuration and provide the helical groove in the shield. The drain wire is generally made of a tinned copper. The grooves and helical portions of the drain wire are wrapped around the shield in a substantially uniform helical manner so as to provide a groove or adjacent helical loops which are spaced from about 0.125 inches to about 0.75 inches throughout the entire length of the cable.
The drain wire and the corresponding grooves hold the shield in tight contact and substantially prevent the shield from shifting or opening during its use. Therefore, the cable provides a substantially low impedance variation throughout its length.
Because of this construction, the cable can be flexed around corners without substantially changing the impedance variation. The grooves or corrugations act in a similar manner as an accordion. When the cable is bent, the grooves on the top of the bend tend to straighten out and the grooves on the underneath side compress. When the cable is straightened out the grooves go back to their normal state. This can be repeated over many flex cycles and the skill will remain in its substantially closed state due to the holding feature of the helical drain wire which provides helical loops and corresponding helical grooves spaced at predetermined intervals.
The construction of the shield substantially maintains the impedance variation at acceptable levels throughout the cable.
The cable jacket is sized such that when it is placed over the shield and helical drain wire, the jacket tightly contacts the shield portions between the drain wire helical loops and contacts the drain wire. The outer appearance of the jacket has a helical protrusion which corresponds to the helical contour of the drain wire. The jacket is the normal type of jacket used for cables, i.e., polyvinyl chloride, fluorocopolymer, Teflon, Natural Flamarrest, polyethylene, polypropylene.
The twisted pair conductors are the known conductors used for the intended purpose. The preferred twisted pair for high transmission cable is the Belden 350 which is produced by Belden Wire & Cable Company. This is a twisted pair which generally has a dielectric insulation of polyvinyl chloride, polyethylene, polypropylene and fluorocopolymers. The conductors are copper strands, solid copper, or tinned copper. Of course, other suitable conductor material may be used. The Belden 350 has the dielectric layer surrounding each conductor and being joined along the entire length of the dielectric.